"I have a funny feeling," said Richard Maltby, Jr., musing about the body of work his words and David Shire's music have created, "when our entire oeuvre is put together, there'll be one great Seven Ages of Man. We've written about birth; we've written about adolescence; we've written about getting older; we've written about young love. If you put it together, that's a major-league life."

And we're only talking two Broadway shows here. In 1984 they made their Baby step to Broadway. Baby was a show about three different age groups of couples who turn ready or not, like it or not into parents. Now, this month at the Shubert, they're making the Big, two-in-one leap from adolescence to young manhood. Just think how far they'll go if they ever make it out of the B's!

Big began life as an original (!), Oscar-nominated script by Gary Ross and Anne Spielberg back in '88 when body-switching movies were at high tide. It was the one that worked primarily because it pitched its adolescent fantasy into a universal ball park. Via some mystical mumbo jumbo, its 12-year-old Josh Baskin gets his wish to be older well, half a wish: He's physically an adult but, inside unchanged, mentally and emotionally still an adolescent. In that grown-up disguise, he goes far up the corporate ladder, putting his advanced sense of fun to work creating products for a toy company.

Another reason for Big success: It was Tom Hanks's first Toy Story and first Oscar nomination. "Hanks's performance had an enormous amount to do with it, yes, but Daniel Jenkins fulfills exactly the same thing," said Mike Ockrent, the Me and My Girl and Crazy for You director who's staging this musicalized Big. Jenkins barged on to Broadway in 1985 via Big River, a full blown star and Tony-nominated Huck Finn. In a sense Josh is a logical, latter-day extension of that role another innocent at sea with his turbulent times. Ockrent turned the notion over in his noggin and pondered it, bemused: "Huck Finn gets Big."

But not too big. A gangly, awkward boyishness still resides happily in Jenkins even at 33. "It's not a stretch, let's just say that," he grinned. "My child is very much alive." True to those words, the actor's favorite moments onstage are tripping the light fantastic on a massive set of piano ivories with the toy-company president (Jon Cypher).

"Everyone expects me to say that, but it actually is the most fun," Jenkins admitted. "It's like two-handed piano, like playing 'Heart and Soul' or 'Chopsticks' when you were a kid except it's grownups, and it's huge, and you're playing it with your whole body and doing it with somebody els

It was the most exuberantly memorable scene in the movie, and it's the show-stopping centerpiece of the musical its "Heart and Soul," maybe even its soul. Instead of tapping out that ditty on the 88s (as was done in the movie), Maltby and Shire came up with a couple of originals, "Fun" and "The Time of Your Life" which is just what Jenkins, Cypher and the whole company have and give.

Getting these numbers choreographed by Susan Stroman an inexhaustible source of fun (witness her fancy, Tony-winning footwork for Show Boat and Crazy for You) is tantamount to license to kill and laying 'em in the aisle nine deep.

"I was really worried because I'm not a big dancer, unfortunately," Jenkins admitted, "but Susan's really generous and patient. It's like improvisation with music, what she does with dance. And it's an amazing thing to behold."

For the endlessly inventive Stroman, such fun was hard to come by. "Tedious" was her word: "Every key they step on is the correct key my dance arranger, David Krane, plays on the piano. If he hits a jazz chord, they do a jazz chord with their feet. It was a two-week process just to get the correct keys."

The wealth of wit and nuance in Stroman's choreography is, like Ockrent's direction, character driven. "Since this is a musical comedy, the characters have a great deal of humor inside their bones," she insisted. To nail Josh's body movements, she spent a year "obsessively" watching children. "At that age they can't articulate. They do it through movement. I invented a dance for them that was like signal dancing or game playing, with elements of hip-hop and break dancing it's not pure anything. I had Dan learn the dance with the kids and watched him to make sure his body moved like a 12-year-old's."

The quest for elusive fun is a common denominator in the show, knowing no age barriers. The toy-store magnate is as susceptible as the next guy, contended Cypher: "He's a guy who forgot how to have fun, and through the good offices of this young man, becomes reacquainted with the concept, and fun's what made him successful in the toy business in the first place. Actors are like that. We're players in plays, but it's all very serious my billing, my contract. Is the show going to make it? Will the sitcom run? We forget it's fun. That's why we got into it because it was fun."

Crista Moore, who plays Jenkins's lady-executive love-interest, seconded that motivation: "When you meet my character, she has fallen into a pitfall. She's a woman in a man's world a man's corporate business world. It's about power and money and competing and getting ahead and not losing your place but losing the joy in your life, having your values shift kinda without realizing it."

The other prominent female in the show Barbara Walsh, who plays the fretful mom the 12-year-old Josh leaves behind made no bones about why she wanted to do the role: "Stop, Time," a ballad with deep emotional underpinnings tossed to her in the second act. "If the point is to communicate, I think I can do it well with that song," she said. "It's really soaring and glorious, not just musically and lyrically. I think the sentiment is lovely and will touch home."

"Touching home" was John Weidman's prime mover in doing the musical book. "There's something about kids today that's worth exploring and writing about," he declared, "particularly a kid who is on the verge of making a transition from boy to young man. This is where Josh Baskin is at the beginning of the story, and it's the frustration of being in that position that provokes his wish and sends him spinning off into this adventure. I think kids in our society are at risk and it is very much about that particular transition, which is made much too early I think. The show is meant to be fun but it will be, after all is said and done, moving."

The unexpected poignancy in the piece had a few actors actually crying in rehearsals, said Maltby. "The interesting thing that happened to this show why I feel it's on a really good course is I don't think anybody expected it to be the show it's turning out to be. When you're working on a show, pursuing the truth of it, it becomes something it wants to be not necessarily what you expected or even what you predicted. It's a more emotional show than we thought it was going to be. It strikes all the chords of growing up.

"Growing up's about winning and losing gaining something and giving something up and it's a hard passage. You don't stop, either. We're doing it at our age. Life is a combination of those two things what you gain and what you give up along the way and, in its mythic sense, Big is about a kid who skips the middle part, who just goes there. He gets a chance to glimpse it but knows he wouldn't be that person if he didn't go back. It's a fantasy and a comedy, but underneath it all are the basic issues of life that hit you in the heart."